1. Field of the Invention
The present invention relates to an oxide ceramic material containing alumina (aluminum oxide, Al2O3) as a principal component, a multilayer ceramic substrate employing the same and having a conductive body therein, a ceramic laminate device, and a power amplifier module employing the same.
2. Related Background Art
Multilayer wiring substrates on which semiconductor ICs are mounted are classified roughly into organic substrates mainly made of organic materials such as glass epoxy, and inorganic substrates mainly made of ceramics such as alumina or glass. Inorganic substrates are used widely, which generally are characterized by high heat-resistance property, high heat conductivity, low thermal expansion, low dielectric loss, and high reliability.
The inorganic multilayer substrates are classified roughly into a HTCC (high temperature co-fired ceramics) type and a LTCC (low temperature co-fired ceramics) type. The HTCC type is formed using Al2O3, AlN, BeO, SiC—BeO, etc. as a base material. These ceramic materials are manufactured by molding a raw material in powder form and thereafter sintering the same at a high temperature of 1500° C. or above. Therefore, as a material for a conductive body formed in a multilayer substrate, Mo or W having a high melting point is used, but Mo and W have a defect of a high resistivity as a conductive body. On the other hand, Ag and Cu having low resistivities have low melting points and hence are molten in sintering at a high temperature. Therefore they cannot be used as a wiring conductive body for use in a HTCC type substrate. Further, the sintering temperature of 1500° C. or above is a significant loss of energy.
Then, a substrate is provided by modifying a ceramic material such as alumina so as to have the sinterability at a low temperature at which Ag or Cu is not molten; namely, the LTCC type substrate. The LTCC type substrate is made of a ceramic material modified by mixing a large amount of a glass material of a low melting point therein at a ratio of about 50 wt % so that the sinterability at low temperature is achieved, and examples of the material compositions include, for instance, borosilicate lead glass+alumina, borosilicate glass+cordierite, and other various compositions. These can be sintered at a temperature of 1000° C. or below, and therefore, they can employ Ag or Cu having a low resistivity as internal conductive body. Therefore, as the inorganic multilayer substrate, the LTCC type has been used mainly, as compared with the HTCC type. The LTCC material allows capacitors, inductors, and the like to be formed therein easily, and is also used as a ceramic laminate device having a function beyond the function as a mere wiring substrate. However, the LTCC material containing a large amount of such glass loses excellent characteristics such as high heat conductivity and high mechanical strength that alumina originally possesses, since glass has low heat conductivity and low mechanical strength.
When the heat conductivity of the substrate decreases, in the case where a heat-radiating element such as a power amplifier is mounted, a considerable temperature rise occurs due to low heat radiation, thereby causing the element to become unusable. This tendency is remarkable particularly in portable machines that are required to be downsized. To solve this problem, a scheme for forming vias for heat radiation made of a metal conductive body, that is so-called thermal vias, in a portion of the LTCC material below the element mounted is used. However, as a mounting density increases with further downsizing, the provision of thermal vias decreases the freedom in designing, thus hindering the downsizing.
As alumina-based materials whose sintering temperatures are decreased without the use of glass, an alloy of aluminum and bismuth formed in an amorphous state by rapid cooling and subsequent oxidation (refer to the patent document 1 below) and a material obtained by adding manganese oxide and vanadium oxide together to alumina (refer to the patent document 2 below) are disclosed.
Patent document 1: JP 3(1991)-271115 A
Patent document 2: JP 11(1999)-157921 A
However, there are the following problems. An amorphous oxide obtained by oxidizing the alloy of aluminum and bismuth after rapidly cooling the same takes a specific form such that bismuth is dissolved in a solid solution form in alumina, or a specific form such that bismuth as an essential component as well as another elements are dissolved in a solid solution form in alumina. Besides, as for the manufacturing method, they cannot be prepared by a normal ceramics process of only mixing material powders, and molding and sintering the same, and the molding of the same requires simultaneous application of temperature and pressure. Therefore, the method of manufacturing the same is not suitable for mass production. On the other hand, a material obtained by adding manganese oxide and vanadium to alumina involves environmental problems since the added materials are toxic substances and added amounts thereof are several percents, which are not small amounts. Still further, since the reactivity of vanadium oxide with silver is high, when the internal conductive body is silver, the materials tend to cause reaction upon sintering.